Cooler temps, changing landscapes and thinner crowds. These are a few of the benefits of traveling during spring and fall — periods known in travel lingo as “off” or “shoulder” seasons because, well, they aren’t summer. Until recently, only savvy travelers took advantage of off-season exploration, particularly in the fall. Now, summer hotspots that once saw tourism taper off after August are brimming with visitors into autumn. Traditional beach trips to places like the Hamptons are up 78% year-over-year in September, according to Booking.com, while Expedia reports searches for rooms in Dublin have increased 35% in fall over summer. The French Riviera is now as busy in September as it is in August. But this newfound fall frenzy comes at a cost: Off-season travel costs are climbing.